When the Asynchronous Transfer Mode (ATM) standards were being formulated the Virtual Path Identifier (VPI) field was intended to serve two purposes; to provide an independent non-interacting Transport Layer, and to enable an access sub-network which provides the equivalent functions to Concentrators and Grooming Switches in the Public Switched Telephone Network (PSTN).
Multiplexers in the PSTN provide a one-to-one bandwidth relationship between the subscriber line and an upstream multiplex; a multiplexer is a managed entity. Statistical Multiplexers, used in ATM and other packet-mode networks are also managed entities but they do provide statistical gain, that is the total upstream bandwidth occupied can be less than the sum of the peak bandwidth of the inputs; this is because the process of merging many sources onto one bearer results in a smoothing of the distribution and hence, the peaks are less significant. A statistical multiplexer therefore provides a similar function to a concentrator in the PSTN.
Switches which operate on the VPI field form a managed network and statistical multiplexing (concentration) and grooming are managed network functions, thus routing upstream on the VPI field in the access network is an obvious choice. The choice of VPI for routing downstream is less obvious but there are two reasons why this should be so; firstly, the same switches are likely to be used for handling routing upstream and downstream and to route on VPI upstream and VCI downstream would affect the complexity, and hence the cost, of the switches. Secondly the downstream path would, in many cases, make connection to the equivalent of an S-bus serving several terminals, requiring many virtual channels which can also be contained in one virtual path.
Another feature often specified for PSTN multiplexers and concentrators is own-unit switching capability, usually in order to provide a limited service if the upstream link(s) fail. In the above scenario for Broadband-Integrated Services Digital Network (B-ISDN) access, since there must be VPI addresses for all the ports in the downstream direction, and since switches rather than simple statistical multiplexers are used in the access network in order to provide grooming, all that is necessary to provide an own-unit switching capability is to place the "upstream" (grooming) ports and the "downstream" (subscriber) ports in a common address group.